Most style guides call for spelling out numbers less than 10, and using numerals for those 10 and over. While reading a magazine today, I saw the phrase nine out of 10, and it struck me as wrong even though it technically adheres to the standard. It seems like an exception is called for in this case, instead of the distracting mixing of word and numeral. Do style guides address this issue?

Yes, according to the Chicago Manual of Style I have read recently, the numbering should most importantly be consistent. Then, I believe that it is generally advised to use numerals only for numbers 13 and greater.

Edit: The Chicago Manual of Style, 15th edition, 9.3:

In nontechnical contexts, the following are spelled out: whole numbers
from one through one hundred, round numbers, and any number beginning
a sentence. For other numbers, numerals are used.

Thirty-two children from eleven families were packed into three vans.

9.6 An alternative rule:

Many publications, including those in scientific and financial
contexts, follow the simple rule of spelling out only single-digit
numbers and using numerals for all others. This system should be used
with flexibility so as to avoid such awkward locutions as “12 eggs, of
which nine were laid yesterday.”

Style guides often suggest using words for numbers below ten, digits for higher values. As @Henry points out, mixing style is not endorsed, but increasingly people are forgetting (or never knew) that caveat...